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French researchers have measured the amazing ability of spiders to use camouflage to simultaneously evade predators and ambush prey  despite the fact that predators and prey see things quite differently.

Marc Théry and Jérôme Casas report in this week's Nature on their experiment with a crab-spider, which is able to conceal itself in both pink and yellow-petalled flowers.

"Crab-spiders (Thomisus onustus) positioned for hunting on flowers disguise themselves by assuming the same colour as the flower, a strategy that is assumed to fool both bird predators and insect prey," write the researchers.

"But although this mimicry is obvious to the human observer, it has never been examined with respect to different visual systems."

Birds are able to detect more colours than bees and so their colour vision has a different sensitivity, making a precise colour match difficult.

Théry and Casas devised an experiment to investigate how well spiders managed these different visual systems. They used a method called spectroradiometry to measure the light reflected from a spider sitting on different flowers, and then a reconstruction system to 'see' how the spider appeared to the different visual systems of birds and bees.

They measured the colour of the spider against different parts of the flower, showing that the spider was able to match the colour of its background precisely. From birds' and bees' point of view, there was no colour contrast and the spider was essentially invisible.

The catch, however, is that birds and bees also use black and white vision to detect the brightness of moving objects in some cases. When the researchers tested the spider's disguise under this regime they found that it could be seen by both prey and predator.

Previous research has shown that another species of crab-spider is able to match both colour and brightness contrasts when seen from the bee's point of view, leading the researchers to conclude that such "aggressive mimicry may vary from species to species".

Dr Jochen Zeil of the Visual Sciences Group in the Australian National University's Research School of Biological Sciences said the research raises as many questions as it answers.

"How do spiders chose the backgrounds to sit on, how do they match the colour, and how is it that some match colour but not brightness, and some are able to match both?" he said.

"It may be a reflection of the limits of what the spiders can do or the pressure the spiders are under. Perhaps different predators have different strategies, for example?"